Yes, this is true. Genuine parallelism will absolutely be a problem there - similarly to how it is for user-level record/replay tech. And serialising is going to have more impact when you're recording a whole OS vs just one application.

Microsoft's TTD (a user-level tech) does allow genuine parallelism, I believe. But my understanding is that it has slower (though still very impressive) single-threaded performance as a result, so there's a tradeoff. But perhaps you could do something more like that.

We believe it's possible to handle parallelism in record/replay tech while still having good single-threaded performance - but it implies a much more complex implementation to do so safely.